Because they are electrically neutral, neutrons and gamma-rays have been traditionally detected using indirect means. However, typical indirect techniques of the prior art, for neutrons, for example, while able to measure count rate, provide little, if any, information on the neutron's energy or the location of the neutron's source. This lack of information limits the usefulness of prior art detectors in a number of applications, including the detection of special nuclear material (SNM). These materials—specifically uranium and transuranics—emit neutrons via spontaneous or induced fission, which neutron emissions are unique to fissionable material.
While position sensitive neutron detectors have been described in the prior art, such as the COMPTEL as described in J. Ryan, et al., “COMPTEL as a Solar Gamma-Ray and Neutron Detector,” presented at Data Analysis in Astronomy; 1992, the active areas of these prior art detectors were typically a flat surface, with a limited field of view. The radial symmetry of the detection of the present invention is a desirable feature in several applications. In space-based orthogonal to the direction to the Sun. Therefore, a flat-surface detector has a time-dependent sensitivity to solar events, which is undesirable when detecting time-varying neutron or gamma-ray fluxes, such as the ones from solar flares. In another important application, the search for SNM emitting neutrons, a radially symmetric detector placed in any area (e.g. a storage warehouse or loading dock) provides a complete 360° wide scan with no need to change its orientation.
A cylindrically symmetric imaging neutron detector described in the prior art is described in U.S. Pat. No. 5,345,084. However, the detector therein is based on count rate rather than measurement of individual neutrons and, as a result, provides no information on neutron energy and no means to identify gamma-rays. With respect to the coordinate system in FIG. 1, the detector of the prior art can determine the azimuthal angle φ of the neutron source (with relatively poor accuracy, Δφ˜30°-45°), but is unable to measure the zenith angle θ, making it impossible to locate point sources.